You can get a (usually free) transcript of your return at the link below.
https://www.irs.gov/individuals/get-transcript
If you need an actual copy of your return, a fee is involved and you will need to prepare and mail Form 4506.
If you prepared the 2016 return in Online TurboTax and need the actual return, log in and go to the section "Your Tax Returns & Documents". Expand that and choose 2016. Do you find evidence of the return with a link to download/print the PDF? If not, we can tell you how to look for multiple accounts.
Or do you find the link to download/print the 2016 return, but access is locked and it asks you to pay? If so, we can tell you how to upgrade a 2018 return and pay for PLUS (19.99) to unlock all prior returns in that account.
tagteam also told how you can get a free 2016 tax return transcript from the IRS.